There are no fast, sensitive photodetectors with a narrow but broadly tunable bandwidth for applications including spectral sensing and situational awareness. Considerable effort has been made in this direction, including efforts by the U.S. Air Force Research Lab (AFRL), in the development of photodetectors based on tunable plasmon resonances in the two-dimensional electron gas (2DEG) of grating-gated high electron mobility transistors (HEMTs). However, the detectable wavelengths have been limited to the low THz and mm-waves due to practical limitations on maximum sheet charge density and minimum grating period. Moreover, the strength and sharpness of the resonant absorption is low at room temperature due to limited electron relaxation times, so that such devices have required temperatures below about 20 K for proper operation.